Both a noun and a verb, filk refers to fan-written, fan-sung songs about science fiction or fannish themes and to the events, or filksings, where such music, known as filksongs, are performed by filkers.

Once performed in casual sing-alongs at fan-club meetings and in the stairwells and spare corners of cons, filksinging (also known as filking) has evolved into a highly organized system of concerts and “bardic circles” whose participants have little patience with the musically unskilled. However, a few filkers, such as Filthy Pierre, still conduct sing-alongs.

Although filk began as fannish parodies to the tunes of mundane songs, the meaning of filksong has greatly broadened since Fancyclopedia 2 was published. Today, filk songs may be either parodies or completely original songs, and they encompass a variety of styles. The criterion for a filk song is its place or the place of its creators in fannish culture rather than adherence to any musical approach. However, by definition, they are not instrumental music.

Non Filk

Sf-related songs by mainstream figures who aren't closely involved with fannish culture (e.g., the music of Weird Al Yankovic, Jonathan Coulton, The Rocky Horror Show, etc.) aren't considered filk, even if they have SF themess; if widely adopted by filkers, though, they're sometimes called "found filk."

Filking isn't the only kind of music at cons. Some conventions hold a dance with DJs and, particularly at Minicon, some musicians get together to perform mundane music.

Almost any dance at an sf con will include Rocky Horror's "Time Warp," as surely as the "Hora" will be danced at a bar mitzvah. (At Sydney Krause's bat mitzvah, they played both, and at the first strains of the "Time Warp," Bill Higgins quipped, "Ah, the folk song of my people."}

LeeJ misspelled folk song in a piece he’d submitted to SAPS entitled "The Influence of Science Fiction on Modern American Filk Music." The article was about supposed sf incidents in folk songs, but actually discussed a number of thoroughly smutty songs, taking various metaphors in them as if they were meant literally. Wrai Ballard, the OE of SAPS, decided the piece might run afoul of censorious postal authorities, so he didn't run it in the mailing, but he noted the typo of “filk” for “folk” and mentioned it to a lot of fans. Not long after that another SAPS member, Karen Anderson, took LeeJ’s typo and defined it as musical parodies written by sf fans. These origins explain why some people believe filksinging to be an abbreviation for "filthy folk singing."